I can't comment on the LATCH issue in the Volvo S60, but for a 2003, it should have LATCH anchors. Is there any chance it's called ISOFIX or UAS in the manual?
As for installing the carseat in your dh's car correctly... You can still do it with the traditional method of installation by using seatbelt instead of LATCH. LATCH is supposed to be easier to install, but not always the case. And the middle back seat usually don't have LATCH anchors, so if carseats installed in this position usually has to use the seatbelt anyway.